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Understanding the Changing Clinoform Architecture of the Late Miocene Cruse Formation, the First Clastic Wedge of the Paleo-Orinoco Delta Onshore Trinidad

Abstract

The paleo-Orinoco delta deposited a sediment prism >10 km thick and >200 km wide since the Late Miocene in the Southern and Columbus Basins, Trinidad. The Cruse Formation was the first clastic wedge of the paleo-Orinoco on Trinidad with deposits > 2500 ft thick. Its shelf-margin growth was built by repeated cross-shelf, transgressive-regressive transits (>100km) driven by glacio-eustatic cycles on the 100 k.y. Previous studies and some rare offshore publications provide an understanding of various segments of the depositional system and focused primarily on regime change of onshore Trinidad deposits. However, a single composite picture for any one clinoform from its proximal to distal reaches of the Orinoco wedge through the main fairway from the Southern Basin to the Columbus Basin, has never been built. This study provides a widow into the first clastic wedge, the Cruse Formation. 2D seismic, 54 well logs and outcrop observations were integrated to build a composite picture of the clastic wedge. From well log correlations 9 main clinothems have been identified with an estimated clinoform slope of 2º, and distance from shelf edge to base of slope of ~5 km. The oldest clinothems (Units 1-2) have oblique geometries, lack well-developed topsets, and are associated with up to 200 ft basin floor fan deposits. This trend suggests that initial stages of clinoform development occurred when short periods of icehouse eustatic sea-level fall outpaced subsidence rates, leading to an initial flat trajectory and delivery of sand to the deepwater. Wholesale shelf-edge collapse and canyon features seen in outcrop is evidence that conduits for sediment dispersal and bypass surfaces for Cruse basin floor fans exist. Units 3-7 show sigmoidally shaped clinothem geometries and have strong shelf accretion, with rising shelf-edge trajectories and much thinner slope and associated basin floor sand deposits (30 ft thick). Outcrop observations of these units indicate a strongly wave-dominated shelf-edge environment which are less efficient in delivering sands to the basin floor. The youngest group of clinothems (Units 7-9) once again show increasingly oblique geometries, and flat trajectories, similar to Units 1-2. Ultimately, understanding these trajectory changes across the shelf will help to predict bypassed sands and their possible delivery to deep-water which can be used as an analogue for subsequent paleo-Orinoco clastic wedges.